1. Field of the invention
This invention relates to a semiconductor laser array device.
2. Description of the prior art
Semiconductor laser devices having a single lasing filament structure can only produce 80 mW laser lights at their best. In order to produce high optical output power, semiconductor laser devices, in which a plurality of lasing filaments are disposed in a parallel manner on a single substrate to achieve an optical phase coupling between the adjacent lasing filaments, have been studied.
As shown in FIG. 6, in the case where a plurality of semiconductor lasers R are disposed in a parallel manner with an optical phase coupling between the adjacent semiconductor lasers and the individual semiconductor lasers R have the same gain, they tend to oscillate laser lights with a 180.degree. phase-shift therebetween as indicated by curve 2 in FIG. 6 rather than with a 0.degree. phase-shift therebetween as indicated by curve 1. This is because the optical intensity distribution is in accord with the gain distribution in the 180.degree. phase-shift mode rather than in the 0.degree. phase-shift mode, resulting in high oscillation gain.
The far-field pattern of laser lights with a 0.degree. phase-shift therebetween exhibits a single peak so that the laser lights can be concentrated into a single spot by means of optical lenses, whereas the far-field pattern of laser lights with a 180.degree. phase-shift therebetween exhibits dual peaks so that the laser lights cannot be concentrated into a single spot by any optical lens. These semiconductor laser array devices producing laser lights which cannot be concentrated into a single spot are inconvenient for optical coupling with other optical systems, and cannot be used as light sources for optical discs, etc. Thus, semiconductor laser array devices producing laser lights with a 0.degree. phase-shift therebetween are required.
FIG. 7 shows the waveguide structure of a conventional semiconductor laser array device which selectively oscillates laser lights with a 0.degree. phase-shift therebetween, wherein laser lights with 0.degree. phase-shift therebetween are propagated from the array portion 11 (or 12) to the array portion 12 (or 11) through the branching portion 10 without loss due to optical interference therebetween. However, when laser lights with a 180.degree. phase-shift therebetween are propagated from the array portion 11 or 12 to the branching portion 10, they undergo loss, to a great extent, due to the optical interference therebetween, resulting in little optical output power. Therefore, this semiconductor laser array device can selectively oscillate laser lights in a 0.degree. phase-mode. However, with the semiconductor laser array device having the above-mentioned waveguide structure, it can be observed that a third-type oscillation mode (i.e., an interim-mode) other than the 0.degree. phase-mode and the 180.degree. phase-mode arises with an increase in optical output power. This phenomenon can be explained below with reference to FIG. 7.
When laser lights of the interim-mode 20 are propagated from the array portion 12 to the array portion 11 through the branching portion 10, they are converted into laser lights of a 180.degree. phase-mode 21 without loss. On the contrary, when laser lights of the 180.degree. phase-mode 21 are propagated from the array potion 11 to the array portion 12 through the branching portion 10, some of them are weakened due to optical interference therebetween (wherein the optical power loss is approximately 3 dB) and the others are converted into laser lights of the interim-mode 20.
As mentioned above, although laser lights oscillated in an interim-mode undergo loss due to the optical interference therebetween, the degree of loss thereof is relatively lower than that of laser lights oscillated in a 180.degree. phase-mode. Thus, even though the conventional semiconductor laser array device oscillates laser lights in a 0.degree. phase-mode, when the optical output power increases, the laser array device unavoidably oscillates laser lights in an interim-mode.